The overall reaction for producing potassium sulfate from sodium sulfate and potash can be described as: EQU Na.sub.2 SO.sub.4 +2KCl=K.sub.2 SO.sub.4 +2NaCl
In water, however, the reaction is subject to the thermodynamic constraints of the Na.sub.2 SO.sub.4 -2KCl-K.sub.2 SO.sub.4 -2NaCl/H.sub.2 O system. The data for the system are best represented on a Janecke phase diagram (FIG. 1). The relevant invariant points are:
(a) solution in equilibrium with Na.sub.2 SO.sub.4, NaCl, and glaserite (K.sub.3 Na(SO.sub.4).sub.2); PA1 (b) solution in equilibrium with KCl, NaCl, and glaserite; PA1 (c) solution in equilibrium with KCl, K.sub.2 SO.sub.4, and glaserite. PA1 1. Production of glaserite from sodium sulfate, potash, and Stage 2 liquor; PA1 2. Production of potassium sulfate and Stage 2 liquor from potash, water, and glaserite from Stage 1.
The compositions of the invariant points at 25.degree. C. are given in Table 1 (FIG. 5).
From the phase diagram, it is evident that for any feed mixture of potash, sodium sulfate, and water, pure sodium chloride cannot be removed as a by-product. In addition, a reasonable potassium conversion can only be achieved in a two-stage reaction, via the intermediate product glaserite, consisting of (FIG. 2a):
The glaserite produced in Stage 1 is separated from the mother liquor and introduced to Stage 2. The mother liquor contains substantial quantities of dissolved potassium and sulfate, which generally warrants a recovery operation. While the currently-known processes differ primarily in the scheme used to retrieve the potassium and sulfate, the reaction stages are very similar.
There are numerous problems associated with the solid/liquid separation of glaserite. Since known processes produce glaserite via out-salting, the glaserite produced is characteristically fine. Somewhat larger glaserite crystals can be obtained by out-salting, but this requires a longer residence time and more sophisticated crystallization equipment.
Small glaserite particles filter poorly. Moreover, the amount of adhering mother liquor is greatly increased with decreasing particle size. This mother liquor is rich in sodium (65-86 mole %) and in chloride (75-95 mole %). The bulk of the sodium and chloride introduced to Stage 2 must be dissolved in the effluent liquor. Since at the optimal theoretical operating point (the KCl/K.sub.2 SO.sub.4 /glaserite/H.sub.2 O invariant point), the solution contains approximately 71% H.sub.2 O and under 3% sodium, about 25 kg of excess water must be added in Stage 2 to remove each additional kg of sodium introduced.
`Sodium poisoning` resulting from the liquor adhering to the glaserite increases the feed water requirements (FIG. 3), and hence, the evaporation load in the recovery stage. Energy costs are further increased because of additional heating and cooling costs for the enlarged recycle streams. Equipment costs are is increased correspondingly.
Large glaserite particles are more easily filtered and contain appreciably-less mother liquor.
The use of Glauber's salt (sodium sulfate decahydrate) in the production of potassium sulfate is known in the prior art. It is well-known that the additional water from the Glauber's salt decreases the conversion in the reaction stages and increases the sulfate composition of the Stage 1 effluent. Some cyclic processes cannot be operated using Glauber's salt; others require additional unit operations (e.g. evaporation).
The water-to-sodium sulfate ratio in sodium sulfate solutions is significantly higher than that of Glauber's salt, such that the problem of excess water worsens considerably.
Hence according to prior art, water-containing sources of sodium sulfate are generally subjected to evaporative crystallization to produce the anhydrous salt. The solids are separated from the mother liquor and introduced to one of the conventional process schemes for producing potassium sulfate from potash and anhydrous sodium sulfate.
The production of anhydrous sodium sulfate from Glauber's salt, sodium sulfate solutions and other sources of sodium sulfate is both capital-intensive and energy-intensive. Thus, there is a widely recognized need for a more efficient and more economical way of producing potassium sulfate from these sodium sulfate sources than heretofore known.